Key Takeaways & Executive Findings
- •• • Li|Li cells achieve >2000 h of stable cycling, demonstrating a substantial improvement in dendrite suppression and interfacial stability, critical for extending the operational lifetime of solid-state lithium metal batteries. • • Li|LiNi0.8Co0.1Mn0.1O2 full cells surpass 600 cycles, indicating excellent compatibility with high-nickel cathodes and potential for high-energy-density applications. • • The heterostructured SEI integrates LiF domains (dendrite-suppressing) with Li3N ion channels (ultra-fast ion transport), resolving the trade-off between mechanical robustness and ionic conductivity that plagues conventional LiF-rich SEIs. • • The design is guided by quantifiable descriptors (ionic potential and donor number), providing a rational, transferable framework for engineering SEIs in solid-state polymer electrolytes, moving beyond empirical trial-and-error.
Abstract
The design of solid electrolyte interphases (SEIs) for poly(vinylidene fluoride)-based solid-state batteries has largely focused on solvent-affinity of Li+ to generate robust but ionically sluggish LiF-rich layers, inherently compromising transport kinetics. Here, we establish a paradigm based on quantifiable physicochemical descriptors (ionic potential and donor number) to guide the design of amphitropic ion pairs (AIPs). These AIPs are engineered to simultaneously tailor both the solvent-affinity and anion-affinity of Li+: a solvent-philic cation with high ionic potential (Al3+) first sequesters reactive solvents, clearing the path for a high-donor-number, lithium-philic anion (NO3−) to remodel solvation. This rationally guided, sequential mechanism enables the in situ synthesis of a LiF/Li3N heterostructured SEI, where dendrite-suppressing LiF domains are seamlessly integrated with ultra-fast Li3N ion channels. This design heterogeneity effectively enhances stability and kinetics, yielding a robust and highly conductive interface. Consequently, Li|Li cells achieve >2000 h of stable cycling, and Li|LiNi0.8Co0.1Mn0.1O2 full cells surpass 600 cycles.
1. Introduction
Solid-state polymer electrolytes (SPEs) promise higher energy density and enhanced safety over conventional liquid electrolytes, yet their commercial deployment is stalled by unstable solid electrolyte interphases (SEIs). The strong interaction between Li+ and polar polymer segments or residual solvents, essential for salt dissolution and ion transport, fosters an organic-rich SEI that fails to suppress dendrite growth and exacerbates interfacial side reactions. In poly(vinylidene fluoride) (PVDF)-based systems, residual N,N-dimethylformamide (DMF) reacts with lithium metal to generate aggressive free radicals and HF, causing catastrophic degradation. Conventional strategies to induce LiF-rich SEIs—via fillers or fluorinated solvents—either increase electrolyte density and interfacial defects or suffer from the intrinsically low ionic conductivity of LiF (10−13–10−14 S cm−1), crippling transport kinetics.
This work introduces amphitropic ion pairs (AIPs) to simultaneously modulate the solvent-affinity and anion-affinity of Li+ in a PVDF-HFP matrix. By leveraging quantifiable descriptors—ionic potential and donor number—the authors engineer a sequential mechanism: Al3+ (high ionic potential) sequesters DMF, while NO3− (high donor number) enters the Li+ solvation sheath, remodeling it to be dual-anion-rich. This in situ generates a LiF/Li3N heterostructured SEI, merging dendrite-suppressing LiF with ultra-fast Li3N ion channels. The result is a robust, highly conductive interface that enables >2000 h of Li|Li cycling and >600 cycles in full cells, directly addressing the stability-kinetics trade-off that has hindered PVDF-based solid-state batteries.
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DU Wangkaichen, LOU Ziyu, CHEN Shuoyi, YIN Xunjie, CHI Sijia, YI Xuerui, WANG Li, WU Shichao, YANG Quan-Hong (2026). Amphitropic Ion Pairs Induce a Heterostructured Solid Electrolyte Interphase in Solid-State Lithium Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4116-0
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Frequently Asked Questions
What is the specific role of Al3+ in the amphitropic ion pair mechanism, and how does its ionic potential quantitatively influence the solvation structure?
Al3+ possesses a high ionic potential, which enables it to strongly coordinate with residual DMF solvent molecules. This sequestration effectively removes DMF from the Li+ solvation sheath, reducing solvent-affinity and preventing the formation of organic-rich SEI components. The high ionic potential of Al3+ is a quantifiable descriptor that predicts its Lewis acidity and affinity for electron-donating solvents like DMF, thereby guiding the rational design of AIPs.
How does the LiF/Li3N heterostructured SEI achieve both high ionic conductivity and effective dendrite suppression, and what are the measured performance metrics?
The heterostructured SEI combines LiF domains, which are electronically insulating and mechanically robust, with Li3N domains, which exhibit ultra-fast Li+ transport. This synergy allows for rapid ion conduction while maintaining dendrite suppression. The performance is evidenced by stable Li|Li cycling for over 2000 hours and full cells exceeding 600 cycles, indicating that the SEI effectively balances kinetic and stability requirements.
What are the cycling stability and rate capability of the full cells with NCM811 cathodes, and how do they compare to conventional PVDF-based systems?
Full cells with NCM811 cathodes demonstrate stable cycling at a cutoff voltage of 4.3 V at 1 C for 300 cycles with 75.8% capacity retention, and improved rate performance from 0.2 C to 5 C. These metrics indicate superior high-voltage compatibility and rate capability compared to conventional PVDF-based systems, which typically suffer from severe interfacial degradation at voltages above 4 V.
What is the industrial scalability of this AIP-based approach, considering the use of Al3+ and NO3− salts in PVDF-HFP electrolytes?
The approach utilizes readily available salts (e.g., Al(NO3)3) and standard PVDF-HFP processing techniques, suggesting ease of integration into existing manufacturing lines. The in situ SEI formation eliminates additional processing steps, and the improved cycling stability could reduce lifecycle costs. However, the long-term stability of Al3+ and NO3− under high-voltage operation and potential side reactions with cathode materials require further validation for commercial deployment.
How does the donor number of NO3− influence the solvation structure and SEI composition, and what is the optimal donor number range for this strategy?
NO3− has a high donor number, which enhances its affinity for Li+ and promotes its inclusion in the solvation sheath. This dual-anion-rich solvation facilitates the formation of Li3N in the SEI. The optimal donor number range is not explicitly specified, but the rationale suggests that anions with donor numbers higher than that of FSI− are effective. This descriptor-based approach allows for systematic screening of alternative anions to tailor SEI properties.
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